Coating material for metallic base material surface

ABSTRACT

A spraying material for coating a metallic member, being inexpensive and having superior corrosion and erosion resistance even in high temperature environments, wherein the material is composed of an Fe—Si based compound and comprises an alloy composed of 10 to 35% by weight of Si and the remainder of Fe and inevitable impurities, and further, up to 5% by weight of B may be added to the material in terms of improvement in hardness and adhesiveness of the coating is provided in order to protect components such as heaters and water walls of coal fired boilers and heat exchanger tubes of fluidized bed boilers, etc., from corrosion and erosion. The spraying material is formed into a coating by, for example, high velocity flame spraying or atmospheric plasma spraying.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. application Ser. No.12/398,577 filed Mar. 5, 2009, which is expressly incorporated byreference herein in its entirety.

DESCRIPTION

1. Technical Field

The present invention relates to a surface coating material (thermalspraying material) for a metallic base material, having superiorcorrosion and erosion resistance in environments, such as heat exchangertubes and water walls of coal fired boilers, heat exchanger tubes andwall surfaces of fluidized bed boilers, etc.

2. Background Art

As capabilities a coating material for the aforementioned heat exchangertubes and wall surfaces of coal fired boilers, etc., should possess, (1)that coating has corrosion resistance in high temperature operatingenvironment, low oxygen partial pressure and high sulfur partialpressure, and (2) that porosity of a sprayed coating can be reduced,thereupon preventing corrosive gas from penetrating into an interfacebetween the coating and the base material are cited. At the same time,(3) resisting erosion due to coal ash is required in terms of erosionresistance for the aforementioned coating material. Also, (4) thatcoating is not cracked due to thermal cycles caused byoperating/stopping of a boiler and removal of the slag deposit from thesurface of the coating is required. In addition, since the boilermembers have a wide area and thus a large amount of thermal sprayingmaterial, (5) being superior in cost efficiency is required.

As a corrosion- and erosion-resistant thermal spraying material ofindustrial boilers, Ni base materials such as Ni—50Cr and metalliccarbide material such as Cr₃C₂/NiCr are conventionally used and coatedby a high velocity flame spraying or atmospheric plasma spraying.

Although those materials have high corrosion and erosion resistance,expensive nickel (Ni) and metallic carbide are included.

Further, although a high corrosion-resistant coating can be obtainedwhen the high velocity flame spraying is used for those materials, thereis a problem that a chromic oxide lowers toughness of the coating andthus cracks and spalling are caused by thermal cycle.

Sulfidation corrosion environments of boiler furnace walls feature ahigh temperature at approximately 500° C., low oxygen partial pressureand high sulfur partial pressure. As elements capable of forming astable oxide coating even in such environments, chromium (Cr), aluminum(Al) and silicon (Si) are known.

Oxide is produced due to heat of a spraying flame when a coating isformed with those elements by a spraying method. A thermal expansioncoefficient of oxide is lower than that of a base material metal. In acoating which contains a large amount of oxide, operating/stopping ofthe boiler and removal of the slag deposit from the surface of thecoating cause cracks due to thermal induced tensile stress, whichresults in spalling off the outer surface of the coating. As describedabove, there is a disadvantage that a crack due to thermal cycles tendsto be caused when a sprayed coating of a simple element having corrosionresistance is used.

A high temperature- and corrosion-resistant thermal spraying material asdisclosed in Patent Literature 1 is such that Si particles not more than35% by weight are mixed into a powder having a base materialcomposition. It is disclosed that by making into a powder with acomposition similar to the base material, a difference in thermalexpansion coefficient with the base material becomes small and thus acorrosion-resistant coating superior in spalling resistance can beobtained.

Patent Literature 2 discloses that a material having a main constituentof magnetite (Fe₃O₄ and/or FeFe₃O₄) and added with carbide, nitride,silicide, boride and/or oxide is sprayed on a metallic base material,and a corrosion- and erosion-resistant layer is formed on the basematerial.

Patent Literature 3 discloses that a bond coat layer composed of an Sicompound and a top coat layer composed of SiO₂ are coated on an uppersurface of a heat exchanger tube or fire grate of an air heater, a steamgenerator and the like by a spraying method and further the topcoatlayer is provided with porous sealing, thereby maintaining hightemperature corrosion resistance of the base material.

-   [Patent Literature 1] Japanese Published Unexamined Patent    Application 2005-240106-   [Patent Literature 2] Japanese Translation of International    Application (Kohyo) No. 2003-522289-   [Patent Literature 3] Japanese Published Unexamined Patent    Application 2005-272927

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the invention as described in the aforementioned Patent Literature 1,a thermal spraying material obtained by mixing Si powder and powder witha base material composition is used. In thermal spraying of a mixedmaterial, part of the powder is combined in processing. However, themajority of the coating is maintained at the original composition. Whenthe present invention is applied to a low-alloy steel base material, athermal spraying material composition becomes a mixture of Fe particles,Si particles, and Fe—Si composition. Fe particles are inferior incorrosion resistance. Thus, superior corrosion resistance cannot beachieved by this coating.

In the invention as described in the aforementioned Patent Literature 2,the thermal spraying material is composed of magnetite with addition ofcarbide, nitride, silicide and boride. Although this material possesseshigh temperature and oxidation resistance, iron oxide is reduced intoiron sulfide in environments of high temperature, low oxygen partialpressure and high sulfur partial pressure, and accordingly corrosionresistance cannot be achieved by this coating. Further, the sprayedcoating composed of a mixed material of oxide-intermetallic compoundbecomes porous, and thus a corrosive gas passes through pores in thecoating, corrosion advances on the interface between the coating and thebase material.

In the invention as described in the aforementioned Patent Literature 3,this material has the same problem regarding the porosity in the coatingas the above-described Patent Literature 2. As a result, a sealingtreatment with an inorganic glass material is required in order to fillthe pores within the top coat. The sealing treatment is such that slurryof inorganic glass such as aluminosilicate glass is applied on a surfaceof the sprayed coating, heated at approximately 1000° C. Since thermaldeformation and strength reduction in the member occur due to thesealing treatment, the invention cannot be applied to a low-alloy steelbase material. The same applies to the bond coat layer. Although an Sicompound layer composed of Cr₃Ni₅Si, Fe₃Si and Cr₃Si, etc., is formed bya thermal diffusion method, this treatment also requires heating at hightemperature. When the invention is applied to a low alloy steel, thermaldeformation and strength reduction occur.

Accordingly, it is an object of the present invention to provide acoating material for a metallic base material capable of forming a finecoating with superior corrosion and thermal shock and thermal cycleresistance even in high temperature environments in order to protectmembers such as heaters and water walls of coal fired boilers, heatexchanger tubes of fluidized bed boilers, etc., from corrosion anderosion.

Means for Solving the Problems

The aforementioned object of the present invention can be solved by thefollowing means.

The invention according to claim 1 is a coating material for a metallicbase material surface, comprising an alloy composed of 10 to 35% byweight of Si and the remainder of Fe and inevitable impurities.

The invention according to claim 2 is a coating material for a metallicbase material surface, comprising an alloy composed of 10 to 35% byweight of Si, 0 to 5% by weight of B and the remainder of Fe andinevitable impurities.

The invention according to claim 3 is a high temperature- andcorrosion-resistant member wherein a surface of a metallic base materialis coated by at least one or more kinds of compounds among a group ofiron-silicon compounds composed of Fe₃Si, Fe₂Si, Fe₅Si₃ and FeSi.

The present inventors have conducted research and then acquired fromfindings that a practicable, fine, corrosion- and erosion-resistantthermal spray coating can be obtained by using a spraying material whosemain constituent is an iron-silicon based compound that is a compoundstable and difficult to be decomposed by heat in coating process andwhich is classified into an appropriate particle size.

The present invention is a surface coating material of an Fe-based alloycontaining 10 to 35% by weight of Si. In terms of improvement in erosionresistance, up to 5% by weight of boron, an element lowering a meltingpoint and strengthening the bonding between particles in the coating,may be added to the above material. The spraying material is formed intoa coating by a thermal spraying method as represented by a high velocityflame spraying, atmospheric plasma spraying, etc.

The coating of the metallic base material surface in the presentinvention exerts high corrosion resistance by being formed by an Fe—Sibased compound having high corrosion resistance such as Fe₃Si, Fe₂Si,Fe₅Si₃, FeSi, etc.

A mixed material of powder with the base material composition and Sipowder is used as the spraying material of the Patent Literature 1 asdescribed in paragraph [0029], “In the embodiment, the base material 34is a Japan Industrial Standard (JIS) SUS310S material, and accordinglymixed powder of mixing SUS310S powder and Si powder is used as thespraying material.”

When the Patent Literature 1 is applied to an iron-based base material,the powder becomes a mixture of Fe and Si. Heat quantity is insufficientto completely melt and alloy the mixed powder in the aforementionedspraying method, and a ferrite phase which is poor in corrosionresistance remains. Consequently, corrosion resistance under low oxygenpartial pressure condition cannot be obtained as already described.Additionally, simple Si is high in reactivity, and mainly inclusion ofoxide such as SiO₂ is caused and pores are produced in coating,whereupon adhesiveness of the coating is lowered.

For the above reasons, it is preferable to use a compound (alloy) powderobtained for example by an atomizing or crushing an ingot having theaforementioned composition, as the thermal spraying material of thepresent invention.

An addition amount of Si is more preferably in a range from 12 wt %where ferrite is not produced by heat in terms of corrosion resistanceto 25 wt % in terms of controlling hardness of particles for loweringporosity of coating. An addition amount of B is more preferably in arange of 3 wt % or less in terms of controlling hardness of particles.

A size range of powder for high velocity flame spraying is from 10 μm to40 μm, more preferably 10 μm to 30 μm. On the other hand, a size rangeof powder for atmospheric plasma spraying is from 10 μm to 150 μm, morepreferably 30 μm to 60 μm.

The spraying material of the present invention is not limited to a caseof being composed of only alloy powder having the aforementionedcomposition. As long as the outer surface of the coating is a compoundwith the aforementioned composition in the end, a composition of theoriginal spraying particles, coating process, presence of a bond coatlayer and presence of thermal treatment are not limited.

In addition, the sprayed coating with the aforementioned composition iscomposed of at least one or more kinds of Fe—Si based compounds amongFe₃Si, Fe₂Si, Fe₅Si₃ and FeSi. Since a ferrite phase that triggersdeterioration of corrosion resistance and an oxide phase that triggersdeterioration of thermal shock resistance are hard to produce, superiorcorrosion resistance and thermal shock resistance can be achieved.

A chemical composition of the spraying material according to the presentinvention and its limiting reasons are described.

An Fe—Si based alloy generates an iron-silicon compound such as Fe₃Si,Fe₂Si, Fe₅Si₃, FeSi, etc., and does not generate a ferrite phase byrendering the addition amount of Si at 10% by weight or more. Thoseiron-silicon compounds are stable and able to form a fine SiO₂ coatingeven under low oxygen partial pressure. Accordingly, the compounds cannot only make the coating superior in corrosion resistance but also havea feature of having superior erosion resistance due to their hardness.

When the addition amount of Si is less than 10% by weight, a ferritephase low in corrosion resistance appears on the spraying particles, andcorrosion resistance is reduced. When the addition amount of Si exceeds35% by weight, on the other hand, FeSi₂ which is easily decomposed byheat is formed. If a material containing FeSi₂ is used for coating bythermal spraying, FeSi₂ is decomposed to FeSi and Si phase. Si is easilyoxidized to SiO₂. SiO₂ is poor in bonding strength and is subjected tocracks by thermal cycles or thermal shocks. In terms of thermal shockresistance, the threshold amount of Si is 35% by weight.

Addition of boron (B) to the aforementioned Fe—Si based material allowsfor lowering of a melting point of the spraying particles andimprovement in adhesiveness of the coating. However, the hardness of thespraying particles is increased with the increase in the amount ofboron, and coating formation becomes difficult. Thus, the additionalamount of boron is 5% by weight at the maximum.

The high velocity flame spraying and atmospheric plasma spraying areavailable for coating methods of the spraying material of the presentinvention. In generally, the atmospheric plasma spraying is anunsuitable for hard facing for erosion protection. However, the sprayingmaterial of the present invention provides high corrosion- anderosion-resistance coating.

For coating formation of the material of the present invention, aparticle size range needs to be 10 μm or more in terms of prevention ofstoppage of a spraying gun nozzle by the powder. Further, a maximumparticle size capable of forming a coating is different between bothspraying methods. When a thermal spray coating is formed by the highvelocity flame spraying, an optimum particle size range is preferably 10μm to 30 μm for the purpose of preventing erosion by other sprayingparticles in processing. When a coating is formed by the atmosphericplasma spraying method, on the other hand, the particle size ispreferably in a range of 30 μm to 60 μm in terms of controlling thermaldecomposition of the particles.

The coating method of the spraying material of the present invention isnot limited to the high velocity flame spraying or atmospheric plasmaspraying. The coating can be formed by flame spraying and electric arcspraying as well. Except for thermal spraying, the coating can be formedby a plasma powder welding.

In addition, a cross-sectional microstructure of a high velocity flamecoating of Fe-20 wt % Si of the present invention is shown by an opticalmicroscopic image in FIG. 1.

Effects of the Invention

As described above, the Fe—Si based spraying material exerts excellentcorrosion and erosion resistance when used in corrosive and erosiveenvironments, which are conceivable in coal-fired boiler component, suchas water walls, heat exchanger tubes, etc. Therefore, the material issuitable for a coating of such a component as described above. Inparticular, the spraying material of the present invention is aninexpensive Fe-based material. Installation cost reduction effects canbe obtained as compared with a case where conventional Ni-based powderor metal carbide powder is used.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an optical microscopic image of a cross-sectionalmicrostructure of a high velocity flame spray coating of Fe-20 wt % Siof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, examples of materials of the present invention aredescribed as compared with materials having a composition within andbeyond the scope of the present invention. Spraying materials having acomposition of six kinds as shown in Table 1 were made into alloypowders by an atomizing, and the powder were classified through a sieverespectively. Two kinds of powders in particle size range of 10 to 30 μmand 30 to 60 μm were obtained. Coatings were formed with those powderson a surface of a 0.5 Cr to 0.5 Mo low alloy steel used as a materialfor boiler water walls, by the high velocity flame spraying and theatmospheric plasma spraying.

A specific coating formation condition on the high velocity flamespraying was 250 μm in coating thickness with 15 built-up layers at apropylene pressure of 700 kPa, an oxygen flow rate of 2.0×10⁻⁴ m³/s, ahydrogen flow rate of 2.7×10⁻⁴ m³/s, an air flow rate of 3.2×10⁻⁴ M³ anda powder feeding rate of 28 g/min. DJ-2700 of Sulzer Metco Ltd. was usedfor a spraying device.

A specific coating formation condition on atmospheric plasma sprayingcoating was 250 μm in coating thickness with 20 built-up layers at 800Aand 750V as a plasma condition, an argon flow rate of 7.9×10⁻⁴ m³/s, ahydrogen flow rate of 0.6×10⁻⁴ m³/s and a powder feeding rate of 28g/min. 9 MB of Sulzer Metco Ltd., was used for a spraying device.

TABLE 1 Constituent (wt %) Material C Si Mn P S B Fe O N Example 1 0.01314.78 0.01 <0.001 0.001 <0.01 Bal.  90 ppm 16 ppm Example 2 0.018 19.820.01 0.001 0.001 <0.01 Bal. 110 ppm 13 ppm Example 3 0.032 20.03 0.020.002 0.001 1.03 Bal.  80 ppm 11 ppm Example 4 0.029 29.44 0.03 0.003<0.001 1.02 Bal.  80 ppm 11 ppm Example 5 0.010 34.96 0.026 0.002 <0.001<0.01 Bal.  80 ppm 11 ppm Comparative 0.010 4.97 0.01 0.001 <0.001 <0.01Bal. 120 ppm 20 ppm Example 1 Comparative 0.033 40.50 0.02 0.003 0.001<0.01 Bal. 130 ppm 15 ppm Example 2

Under the above-mentioned condition, a coating could be formed in thehigh velocity flame spraying by using the powder size range from 10 to30 μm. In addition, the coating was impossible when the sprayingparticle size was 30 μm or more. Further, thermal shock test for theobtained coatings with heating at 600° C. and water-cooling for 10 timesis performed. The atmospheric plasma spraying, a crack was observed in acoating with a particle size range from 10 pro to 30 μm. Sincegeneration of SiO₂ and a difference in thermal expansion coefficientbetween oxides and Fe—Si particles is the main cause of cracking.

Table 2 shows particle size range where a fine coating was obtained andTable 3 shows a result of measuring a Vickers hardness of a crosssection of the coating. In addition, a measuring load of the hardnesswas 3N, and an average of 5 times measuring was shown. Compared withHV180 which is the hardness of the base material, sprayed coatings ofthe examples 2 to 4 have a substantially three-fold hardness, so thathigh erosion resistance can be expected.

TABLE 2 High velocity Atmospheric flame spraying plasma sprayingParticle size range 10-30 μm 30-60 μm 10-30 μm 30-60 μm Example 1 ⊚ — X⊚ Example 2 ⊚ — X ⊚ Example 3 ⊚ — X ⊚ Example 4 ⊚ — X ⊚ Example 5 Δ — ΔΔ Comparative ⊚ ⊚ X X example 1 Comparative — — X — example 2 ⊚ . . .fine coating, X . . . crack produced due to thermal shock (coatingformation possible), Δ . . . porous coating, — . . . coating formationimpossible due to spraying particle erosion

TABLE 3 High velocity Atmospheric flame spraying plasma sprayingParticle size range 10-30 μm 30-60 μm Example 1 310 300 Example 2 530400 Example 3 580 440 Example 4 600 580 Example 5 400 450 Comparativeexample 1 220 200

A corrosion test of coatings under conditions of simulating anoperational environment of a coal-fired boiler was carried out. The testtemperature was set to 500° C., and the test time was set to 100 hours.As a simulated corrosive gas composition, a gas with a composition asshown in Table 4 was supplied at a flow rate of 100 ml/min in order tobring into a low oxygen partial pressure and a high sulfur partialpressure and the test was carried out.

Table 4 shows a gas composition used in the corrosion test. The gascomposition was determined by a chemical-thermodynamic calculation ofthe equilibrium composition of high sulfur coal (sulfur content isapproximately 3.3) combustion gas at 1200° C.

TABLE 4 H₂S SO₂ CO CO₂ H₂ H₂O N₂ Composition 0.22% 0.14% 6.3% 13.0% 1.8%10.0% Bal (Volume fraction)

TABLE 5 High velocity Atmospheric flamespraying plasmaspraying Particlesize range 10-30 μm 30-60 μm Example 1 0.23 0.50 Example 2 0.04 0.21Example 3 0.04 0.18 Example 4 0.28 0.30 Example 5 0.35 0.50 Comparativeexample 1 Decayed due Decayed due to corrosion to corrosion 0.5Cr—0.5Mosteel 1.0

Table 5 shows a result of the corrosion test. These values mean a ratioof weight loss per unit area between thermal spray coatings and the basemetal after removal of corrosion product after descaling treatment(specimens were washed by boiling 18 wt % sodium hydroxide +3 wt %potassium permanganate solution and boiling 10 wt % ammonium citratesolution). As for the Fe—Si based spraying materials of the examples, asshown in Table 5, the examples 2 and 3 were one twenty-fifth of the basematerial in the high velocity flame spray coating and approximatelyone-fifth in the atmospheric plasma sprayed coating within the additionamount of Si of the present invention. High thermal shock resistance,coating hardness and corrosion resistance were obtained.

On the other hand, at a content of Si higher than the range of thepresent invention, erosion by spraying particles occurs and the coatingbecomes porous, thereupon being unable to obtain a coating with highcorrosion resistance. Further, at a content of Si lower than the rangeof the present invention, a dense coating could be obtained but wascorroded and decayed by descaling treatment.

INDUSTRIAL APPLICABILITY

The Fe—Si based material for thermal spraying of the present inventionexerts high corrosion resistance since a SiO₂ coating is formed on asurface of a sprayed coating. Accordingly, the spraying material isapplicable to environments of high temperature and low oxygen partialpressure, for example, coal fired boilers for power generation,fluidized bed boilers and incinerators of refuse incinerating plants.

1. A method for preparing a coating for a metallic base materialsurface, comprising an alloy composed of 19.82 to 35% by weight of Siand the remainder of Fe and inevitable impurities the method comprising,providing a coating composition comprising Si, and the remainder of Feand inevitable impurities; and spraying the coating composition onto themetallic base material surface by high velocity flame spraying oratmospheric plasma spraying.
 2. A method for preparing a coating for ametallic base material surface, comprising an alloy composed of 19.82 to35% by weight of Si and the remainder of Fe and inevitable impuritiesthe method comprising, providing a coating composition comprising Si, Band the remainder of Fe and inevitable impurities; and spraying thecoating composition onto the metallic base material surface by highvelocity flame spraying or atmospheric plasma spraying.
 3. A method ofcoating a metallic base material surface, the method comprising:providing a coating composition, comprising a member selected from thegroup consisting of Fe₂Si, Fe₅Si₃ and FeSi; and spraying the coatingcomposition onto the metallic base material surface by high velocityflame spraying or atmospheric plasma spraying, wherein the coatingcomposition comprises an alloy composed of 19.82 to 35% by weight of Si,1.02 to 5% by weight of B, and the remainder of Fe.